The disclosure relates generally to steam turbine technology, and more particularly, to a turbine steam seal system having a valve coupled to a leak off line for controlling a steam flow used to maintain a constant self-sustaining sealing pressure to a turbine. A related method is also provided.
Shaft packings are required to provide sealing of the turbine rotor or shaft between the turbine shells or the exhaust hood and the atmosphere. During normal turbine operations, the end packings can be divided into two distinct groups, pressure packings and vacuum packings. Pressure packings generally prevent steam from blowing out into the turbine room. High pressure and intermediate pressure turbine end packings are generally known as pressure packings. Vacuum packings generally seal against the leakage of air into the condenser. Low pressure end packings are known as vacuum packings. Known steam seal systems largely address these issues by utilizing the steam leaking from the pressure packings to help seal the vacuum packings.
Current steam seal systems are of a single set point sub-optimized design. For example, these designs may provide an unfired guarantee loading with a self-sealing load point (“SSLP”) of about seventy percent (70%). When a steam turbine “self seals”, the terms generally refer to the condition where pressure packing seal steam flow is sufficient to pressurize and seal the vacuum packings. In higher load conditions such as a supplementary firing, however, the pressure packing steam flow going to the steam seal header increases but the vacuum packing requirement may not vary such that the SSLP may be as low as about thirty percent (30%). The additional steam coming from the pressure packings into the steam seal system thus may be dumped to the condenser using a steam seal dump valve without extracting any work. Similarly during low load operations, the pressure packing steam seal flow may be reduced significantly from the design point, but the vacuum packing steam flow requirements again may not vary. In such a situation, the steam seal system may not be sufficient and an extra flow may be required from the throttle steam at a significant loss in performance.